1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a method and an apparatus for driving a liquid crystal display that is adaptive for improving a picture quality as well as reducing a memory capacity.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) controls a light transmittance of individual liquid crystal cells in accordance with a video signal, thereby displaying an image. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying moving images. The active matrix LCD uses thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of liquid crystals such as viscosity and elasticity, as can be seen from Formulas (1) and (2):τr∝γd2/Δε|Va2−VF2|  (1)τf∝γd2/K  (2)                wherein τr represents a rising time when a voltage is applied to a liquid crystal; Va represents an applied voltage; VF represents a Frederick transition voltage at which liquid crystal molecules-begin to manifest a tilting motion; d represents a cell gap of liquid crystal cells; γ represents a rotational viscosity of the liquid crystal molecules; τf represents a falling time at which a liquid crystal is returned into an initial position by an elastic restoring force after a voltage applied to the liquid crystal is turned off; and K represents an elastic constant.        
A twisted nematic (TN) mode liquid crystal has an altered response time due to physical characteristics of the liquid crystal material and the cell gap. Typically, a TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage applied to the liquid crystal cell may change gradually into the next frame before reaching a target voltage. Thus, due to a motion-blurring phenomenon, a moving picture is blurred out on the screen.
FIG. 1 is a waveform diagram representing a brightness variation in accordance with data in a liquid crystal display according to the related art. Referring to FIG. 1, a LCD cannot express desired color and brightness because, upon implementation of a moving picture, a display brightness BL fails to reach a target brightness corresponding to a change of a data VD from one level into other level due to its slow response time. Accordingly, the moving picture suffers from the phenomenon known as motion-blur, and the LCD display quality deteriorates due to reduction of the contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567, which are hereby incorporated by reference, have suggested to modulate data in accordance with a difference in the data by using a lookup table (hereinafter referred to as high-speed driving scheme).
FIG. 2 is a waveform diagram representing an example of a brightness variation in accordance with data modulation in a high-speed driving scheme according to the related art. Referring to FIG. 2, a high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. This high-speed driving scheme increases |Va2−VF2| from the above Formula (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving scheme compensates for a slow response time of the liquid crystal by modulating of a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at a desired color and brightness.
FIG. 3 is a diagram representing an example of a high-speed driving scheme in respect of 8-bit data according to the related art. In FIG. 3, a high-speed driving scheme compares most significant bits of the previous frame Fn−1 with those of the current frame Fn to select corresponding modulated data Mdata from the lookup table if there is a change in the most significant bits MSB. This high-speed driving scheme modulates only some of the most significant bits so as to reduce the memory capacity required for hardware implementation.
FIG. 4 is a block diagram representing a high-speed driving apparatus according to the related art. Referring to FIG. 4, a high-speed driving apparatus includes a frame memory 43 connected to the most significant bit bus line 42, and a lookup table 44 commonly connected to the most significant bit bus line 42 and an output terminal of the frame memory 43.
Frame memory 43 may store most significant bit data MSB during one frame interval and supplies the stored data to the lookup table 44. Herein, the most significant bit data MSB may be the most significant 4 bits of the 8-bit source data, RGB-Data-In. Lookup table 44 compares most significant bits MSB of a current frame Fn input from the most significant bit bus line 42 with those of the previous frame Fn−1 input from the frame memory 43, as shown in Table 1 or Table 2, and selects the corresponding modulated data Mdata. The modulated data Mdata are added to least significant bits LSB from a least significant bit bus line 41 to be applied to the LCD. Table 1 shows an example of the lookup table 44 that compares the most significant 4-bits MSB (24, 25, 26, 27) of the previous frame Fn−1 with those of the current frame Fn and selects the modulated data Mdata in accordance with the result of the comparison.
When the most significant bit data MSB are limited to 4 bits, the lookup table 44 of the high-speed driving scheme may be implemented in accordance with Table 1 and 2.
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400124678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111213151515900123456791112131415151000123456781012131415151100123456789111214151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
TABLE 2016324864809611212814416017619220822424000324864809611214416019220822424024024024016016486480961121281601922082242402402402403200326480961121281601922082242402402402404800164880961121281601762082242402402402406400164864961121281441761922082242402402408000163248801121281441761922082242402402409600163248649612814416019220822424024024011200163248648011214416017620822424024024012800163248648096128160176192224240240240144001632486480961121441761922082242402401600016324864809611212816019220822424024017600163248648096112128144176208224240240192001632486480961121281441601922242402402080016324848648096112128160176208240240224001632484864809611212814417619222424024000016324848648096112128144176208240
In the foregoing tables, a leftmost column corresponds to the data voltage VDn−1 of the previous frame Fn−1 while the top row corresponds to the data voltage VDn of the current frame Fn. Table 1 provides lookup table information in which the most significant bits (i.e., 20, 21, 22 and 23) are expressed by the decimal number format. Table 2 provides lookup table information in which weighting values (i.e., 24 25, 26 and 27) of the most significant 4 bits are applied to 8-bit data.
The motivation for modulating the most significant 4-bit data MSB in this manner is for reducing the memory capacity required for implementing lookup table 44. However, while the 4-bit comparison scheme depicted in lookup table 44 helps in reducing the required memory capacity, it leads to a deterioration of the picture quality due to the non-linearity associated with the fact that rather changing gradually, gray levels jump discontinuously from one value to the next.
In order to reduce the picture quality deterioration, the data width of the modulated data stored in lookup table 44 has to be wide enough, and the input source data needs to have all bits, e.g., 8 bits, compared.
Table 3 is an example of a lookup table that compares 8-bits of modulated data Mdata with all 8 bits of the source data.
TABLE 3        
When the lookup table compares source data using all of the available 8 bits, and the modulated data Mdata pre-stored within the lookup table are 8-bits, since the gray level values change linearly, the picture quality is excellent, whereas the memory capacity increases by leaps and bounds. For instance, if the lookup table compares them by the 8-bits and the modulated data Mdata are 8-bits, the memory capacity of the lookup table is 65,536×8=524,000 bits. Herein, the first term ‘65,536’ of the left side is a product (256×256) of 8-bit source data of the previous frame Fn−1 and those of the current frame Fn, and the second term ‘8’ of the left side is the data width (8-bits) of the modulated data registered within the lookup table 44. Further, if red, green and blue kGB are taken into consideration for implementing color, the required memory capacity of the lookup table is 65,536×8×3=1,5720,000 bits. Accordingly, if the 8-bit comparison scheme is adopted in the lookup table for high-speed driving, since the memory capacity increase, a chip size increases as well as a manufacturing cost.